750 EZSHEHGY AND VISION 



approximately. The quantity radiated actually as measured equatorially, being 



118 



87 watts, the ratio = 1.35 gives the amount of energy lost. 



87 



Calculation of the Luminous Efficiency, and Corrections. — The next 

 step was to determine the ratios 



J-.0.7 y>0.8 



Id\ Id\ 



0.4 , Jo. 4 r 



and = ± 



TdX I Id\ 



0.4 Jo.4 



This ratio has been calculated by many authors, Lux, Nichols 

 and Coblentz (15), Ingersoll, Angstrom (16), Stewart and Hoxie 

 (17), etc. Their methods were different and their results do not al- 

 ways check perfectly, (some varying by more than 50 per cent, 

 for example, those of Lux and Angstrom.) Some of the workers 

 used methods based upon the absorption of one part of the spectrum 

 by water cells in which different substances were dissolved, (copper 

 sulphate, iodine). It has been shown by Nichols and Coblentz 

 that none of these methods based on absorption were reliable. In- 

 gersoll studied the Nernst lamp and published figures of observed 

 luminous efficiency, which vary greatly according to different lamps, 

 and besides, correspond to burners whose consumption was not that of 

 our lamp, (89 watts). Therefore, the energy distribution curve of 

 our burner was plotted by means of a Hilger Infra-Red Spec- 

 trometer.'^ 



Correction for the Non-Normal Spectrum. — The spectrum was cor- 

 rected for the lack of homogeneity of the dispersed beam. Indeed, 

 the refracted rays are contracted in certain parts of the spectrum, and 

 expanded in others, so that, for instance, the same slit opening {e.g., 

 0.25 mm.) covers 0.015 /.c on the spectrum at a mean wave length 

 0.68 fj, (from 0.6725 ^i to 0.6875 m), and as much as 0.266 n, more 

 than 17 times as much, at the mean wave length 2.66 n (from 2.53 n 

 to 2.79 n). This correction was introduced by the consideration of 

 the geometry of the screw motion, (pitch of screw in relation to 



5 Instrument No. 281, Rock salt prism. Angle 59°. 57'. 30". 



